A hydrogen explosion caused by nuclear fuel rods overheating and then coming into contact with water collapsed the outside walls and roof of containment vessel. Hourly radiation leaking from Fukushima is equal to amount permitted in one year, official tells Kyodo. The nuclear agency says that they have detected cesium and iodine outside the unit, which certainly indicates fuel melting at the very least. Noriyuki Shikata, Dir. of Communitions for Prime Minister tweets: Blast was caused by accumulated hydrogen combined with oxygen in the space between container and outer structure. No damage to container. (BradBlog citing several sources)

Reaktor Nr. 3. Teine plahvatus, 12. märts

Containment vessel housing the fuel rods was not breached, per officials. Explosion damaged the reactor building, but not the nuclear containment vessel. The Fukushima facility began using MOX (mixed oxide) fuel last September, becoming the third plant in Japan to do so. MOX fuel has a lower melting point than the other fuels and contains plutonium, making it more volatile and toxic than the fuel used in other reactors. (Also see BradBlog and CLG, collected here.)

Reaktor Nr. 2. Kolmas plahvatus, 13. märts

The emergency cooling system at Unit 2 was damaged during the Unit 3 blast, resulting in the uranium fuel rods at 2 becoming dangerously, and completely, exposed for a number of hours. Containment vessel is damaged, radiation leak feared. Bottom of containment vessel blows. There are several containers around the nuclear fuel for the reactor. The fuel itself is inside rods, which are, in turn, inside a steal and concrete container vessel, currently filled — or partially filled, in this case — with sea water, to try to cool the fuel rods. If the container vessel, “the last line of defense from keeping the radioactivity from being released,” has been damaged by the explosion, fuel and radioactivity could leak into the environment. (BradBlog and CLG, collected here.)